Many goods of modern life are sensitive to moisture and, therefore, exposure to atmosphere having a relative humidity exceeding a specific limit value when these goods are shipped or stored should be avoided. For instance, electronic components can be damaged by exposure to an atmosphere having a relative humidity exceeding a specific limit value, even when the limit value is low and the exposure is only for short period of time.
In order to ensure that quality of such goods is not deteriorated during shipping or storing, it is generally desirable to monitor the relative humidity of the atmosphere to which the goods are exposed.
For this purpose, humidity indicators have been developed. There are generally two types of humidity indicators. One of these humidity indicators reversibly changes colour upon exposure to atmosphere having a specific level of relative humidity. Such reversible humidity indicators typically utilize cobalt chloride as the humidity indicator material impregnated on blotting paper. Military specification MIL-I-8835A describes the details of construction of such indicators and performance requirements. The indicators change colour when exposed to atmosphere having a relative humidity exceeding a limit value and return to its original colour when the relative humidity of the surrounding atmosphere drops below that limit value. Reversible humidity indicators are used to indicate the current condition of a desiccant and/or the current humidity level within the storage container.
The second type of humidity indicator is an irreversible humidity indicator. These indicators are designed to detect whether relative humidity of surrounding atmosphere has exceeded a limit value and provide a visual indication of whether goods stored under said atmosphere have thus been undesirably exposed to atmosphere having a relative humidity exceeding that limit value during any period of the storage time. The irreversible humidity indicator can therefore provide an indication of whether the relative humidity of the atmosphere reached or exceeded the limit value even for limited periods of time, i.e. even if the relative humidity has subsequently dropped below that limit value, for instance when the storage conditions are checked at a later time.
Relative humidity is defined according to the following equation:RH=c(H2O)/csat(H2O)    where c(H2O) is the observed content of water vapour, and csat(H2O) is the content of water vapour at saturation.
If the content of water vapour exceeds saturation, condensation occurs, i.e. liquid water is formed by condensation.
The content of water vapour at saturation is dependent on temperature. The content of water vapour at saturation is the higher, the higher the temperature is, i.e. warm air can take up more water vapour before condensation occurs.
This implies that large changes in relative humidity can occur, in particular when the temperature, at which goods are stored, changes while the content of humidity in the surrounding atmosphere remains constant. If the atmosphere at which the goods are stored has a high content of water vapour and the storage temperature drops, condensation can thus occur. This implies the risk that the stored goods can come into contact with liquid water which usually is highly detrimental.
Under these conditions, a reversible humidity indicator might fail to indicate the temporary presence of high humidity within a storage container even though such high humidity may be sufficient to cause damage to the components present in the storage container.
To maintain the relative humidity of the atmosphere consistently low, shipping containers and long term storage containers usually contain desiccant materials. These desiccant materials remove moisture from the atmosphere up to a predetermined level. The containers are periodically opened to recharge or replace the desiccant materials placed within the container and/or to check the level of relative humidity in the storage container. After replacing the desiccant material, the container is again sealed. In order to determine the relative humidity levels in these storage containers has ever reached or exceeded certain critical levels, irreversible humidity indicators are also often placed within the containers with the desiccant materials. These irreversible humidity indicators can be reviewed at the same time at which the desiccant material is being checked to determine whether a detrimental relative humidity level has ever been reached or exceeded in the shipping container.
One of the first irreversible humidity indicator devices was disclosed in U.S. Pat. No. 2,214,354, which disclosed the use of calcium chloride.
A series of relative humidity indicators, each utilizing a different deliquescent salt, are disclosed in a series of patents including U.S. Pat. Nos. 2,460,065, 2,460,066, 2,460,067, 2,460,068, 2,460,069, 2,460,070, 2,460,071, 2,460,072, 2,460,073, 2,460,074, 2,526,938, 2,580,737, 2,627,505, EP 1 200 819 A0 (also published as WO 01/09601 A1), US 2005/0106735 A1, JP 2007-198828 A and EP 1 705 484 A1.
Some indicator devices are capable of showing different relative humidity levels on the same devices by use of a series of different deliquescent agents that change colour at varying humidity levels, as disclosed in U.S. Pat. No. 2,249,867.
Humidity indicator sheets and devices which contain deliquescent salts and dyes have commonly been used to detect the relative humidity level present within storage containers. See for example, U.S. Pat. Nos. 2,249,867, 4,034,609, 4,150,570, and 4,854,160. Button-type humidity indicators or “plug” humidity indicators are also sometimes used with packaging material and are disclosed, for example, by U.S. Pat. Nos. 2,716,338, 3,084,658, and 4,050,307. Another device for monitoring humidity levels, particularly in poured cement, is disclosed in U.S. Pat. No. 3,680,364.
Another type of irreversible humidity detector is disclosed in U.S. Pat. No. 3,898,172, wherein a deliquescent salt in combination with a dye is disposed on a moisture-impervious, fused substrate. Claimed is a humidity indicator that works between 6% and 20% relative humidity at ambient temperature.
U.S. Pat. Nos. 6,877,457 B1 and 6,698,378 describe an irreversible humidity indicator device comprising several layers and a deliquescent material contained within holes of a carrier member. A coloured absorbent sheet is attached to the back side of the carrier member. A transparent cover sheet is attached to the front side. When the indicator device is exposed to atmosphere having a suitable specific relative humidity, the deliquescent material liquefies and the thus formed solution is absorbed by the coloured absorbent sheet. Thus, the coloured absorbent sheet becomes visible through the transparent cover sheet. Irreversible humidity indicator devices of this type have the disadvantage that their manufacture is relatively complex, in particular because it is necessary to completely fill the holes with the deliquescent material.
A similar irreversible humidity indicator device is disclosed in U.S. Pat. No. 4,793,180. A deliquescent agent (such as an inorganic salt) that is coated with a water-soluble dye is arranged within a recess formed in a porous plastic carrier and covered with a sheet of absorbent material such as a blotting paper. When the deliquescent agent interacts with moist atmosphere, it eventually liquefies and forms a solution coloured due to the presence of the water-soluble dye. This solution is absorbed by the blotting paper that is thus stained and irreversibly indicates that the device was exposed to moist atmosphere. Similar to the humidity indicator device described in the preceding section, the manufacture of humidity indicator devices of this type is relatively complex.
Another irreversible humidity indicator was suggested in JP 61-144570 based on a filter paper impregnated with a deliquescent chemical and a colour forming reagent.
A sensor for indicating a cumulative time period of presence of an ambient relative humidity was disclosed in WO 03/054528 A1, comprising an indicator and a carrier, with the indicator located on one or more limited portions of the carrier. The carrier is a hygroscopic material, allowing a migration of the (colour) indicator in the carrier.
In Patent WO 2009/015873 A2 another cumulative moisture sensor is described, comprising a matrix of a hygroscopic material, a first and second reagent that react only in the presence of water.
In DD 120293, another irreversible humidity indicator device system has been suggested on the basis of an absorbent paper, immersed with saturated salt solution and the dry paper dyed on one side. A paper prepared according to this suggestion cannot be handled, because the salt crystallizes on the surfaces and will exfoliate as soon as moved or touched. Furthermore, it cannot be dyed due to exfoliation.
Yet another humidity indicator device system for the testing of water activity of food was suggested in JP 59-157565 A based on a moisture absorber layer, a moisture permeable sheet and a filter paper impregnated with a dye that discolours with water, e.g. a cobalt salt.
U.S. Pat. No. 4,098,120 disclosed a humidity indicating method and device suitable for indicating a humidity-time history. The invention is based on a deliquescent salt, a liquid absorbing wick, and an indicator means. A solution of a deliquescent salt and an indicator dye is absorbed onto a filter paper reservoir, dried, and the wick, prepared of filter paper as well, is brought and held into contact with the filter paper reservoir by enveloping the wick and the filter paper reservoir with a transparent cover.
It is crucial for the reliable and reproducible indication of elevated relative humidity that an intimate contact between the reservoir and the wick is established and maintained in a reliable and reproducible manner in order to avoid that wicking of the solution of the deliquescent salt is influenced. It was found that the intimate contact between a reservoir and a wick necessary for a reliable and reproducible indication of humidity can not be ensured in the device disclosed in U.S. Pat. No. 4,098,120. Furthermore, it was found that the manufacture of said device is relatively complex. Likewise, it is scarcely possible to reduce the dimensions of the device to such an extent that it becomes possible to employ it in miniature applications such as on individual diagnostic test strips, for instance.
The humidity indicator devices known from the prior art as summarized hereinabove have the disadvantage that their manufacture is relatively complex and therefore requires manual work and/or that the indication of humidity is prone to give erroneous results and hence not reliable. Furthermore, the humidity indicator devices known from the prior art are not suitable for miniature applications as their dimensions can not be sufficiently reduced.
It is a first object of the present invention to provide an indicator material suitable for detecting relative humidity exceeding a specific limit value and/or liquid water in a reliable manner and which can be manufactured in a straightforward manner. The indicator material should be suitable for detecting the temporary presence of liquid water and/or elevated relative humidity, i.e. a positive detection result should be obtained, even if at the time of reading the indicator material any water has evaporated and the relative humidity has dropped below the specific limit value.
The indication of relative humidity exceeding a specific limit value and/or liquid water should be suitable for automatic detection, i.e. that it does not require visual inspection by personnel.
It is another object of the present invention to provide an indicator device comprising an indicator material according to the first object of the present invention.
It is a further object of the present invention to provide a process for the manufacture of an indicator material according to the first object of the present invention. This process should be suitable for automatic production in large quantities.
It is a yet further object of the present invention to provide a process for the manufacture of an indicator device comprising the indicator material that is suitable for mass production by automatic manufacture. This process should be suitable for automatic production in large quantities.